Radioactive steroid precursers are inoculated into log-phase growth cultures of the protozoan, Tetrahymena pyriformis. After varied incubation times, the labeled transformation products are isolated and identified by a combination of chromatographic methods and spectral methods. All of the steroid substrates incubated (androstenedione, progesterone, 17 alpha-hydroxyprogesterone, testosterone, corticosterone, and pregnenolone) were completely metabolized into multiple products. The most frequently observed transformation was the reduction of a 3-ketone to a 3 beta-hydroxy. This was confirmed by the isolation and identification of 4-androstene-3 beta, 17 beta-diol as a product of the testosterone incubation. Identification of other metabolites and isolation of the apparent beta-hydroxysteroid dehydrogenase enzyme system is being continued.